Student or Learner
Is there any plural for extra class?
Let's say there are five subjects are held outside school hours. Should we say 'I have extra classes this week"?
That means we should say, ' i have an extra class', right?
It works the same as any plural. An adjective doesn't make a difference. The plural of "class" is "classes", so if you have more than one class, you use "classes", regardless of adjectives or other surrounding grammar.
PS: Naturally, if you already have one class, and you get another class, then you have two classes; but you don't have two extra classes.